ABSTRACT Introduction Music therapy has growing evidence for its effectiveness in mental health. People with mental health conditions often face significant barriers in obtaining personal valued social roles and feeling a sense of belonging to their community. Overcoming these barriers is an important step in the recovery process. This pilot study investigated whether long-term group music therapy might have an impact on participants’ social skill development, group cohesion and expression of emotional states. Method The study was an exploratory, retrospective, quantitative, longitudinal single-case study (N = 8). Five video recorded sessions were selected and micro-analyzed by independent raters, who rated the Group Environmental Scale (n = 8) and Individual Behavior Observation Categorization scale (n = 4). Statistical analysis was carried out to identify trends over time. The study was registered in ClinicalTrials.gov (NCT04435405). Results A longitudinal improvement was found in the group domains of Relationship (1.6-fold), Personal growth (1.9-fold) and System maintenance (1.5-fold) along the 9-month follow-up period. On the individual level, an improvement was found over time in social skills and group cohesion (4.85-fold), affect (3.15-fold), and musical activities performances (19.9-fold). Discussion The study demonstrated longitudinal improvement trends in social skills, group cohesion and expression of emotional states (affect) in the group as a whole and in each of the four individual assessments. Future studies which will include a larger sample and longer follow-up periods are needed.